Tribology, the study of contacting solid surfaces in relative motion, is applicable to every mechanical device and operation. Examples of productive wear are writing with a pencil, machining, and polishing. Examples of productive friction are brakes, clutches, driving wheels on trains and automobiles, bolts and nuts. Examples of unproductive friction and wear are internal combustion and aircraft engines, gears, cams, bearings and seals. According to some estimates, losses resulting from ignorance of tribology amount to roughly 6% of the U.S. gross national product, and approximately one-third of the world's energy resources in present use represent friction in one form or another. The science of tribology therefore seeks to minimize and eliminate losses resulting from friction and wear at all levels of technology involving moving contacting surfaces.
Magnetic recording is one area of modern technology which suffers from unproductive friction and wear. Magnetic recording is extensively used in audio, video, and digital applications in the form of tapes and disks. The industry is expected to grow by a factor of five or more in the next decade. This growth will be accompanied by dramatic improvements in the technology, and the potential exists for magnetic-recording densities to improve by at least one order of magnitude. Magnetic-recording is accomplished by relative motion between a magnetic head and a magnetic medium. Types of magnetic media for digital recording are: flexible media (tapes and floppy disks) and rigid disks. Physical contact between head and medium occurs during starts and stops ind hydrodynamic air films develops at high speeds. Flying heights (mean separation between head and medium) are on the order of 0.1 micrometer comparable to surface roughness of the mating members. The need for higher and higher recording densities requires that media surfaces be as smooth as possible and flying heights be as low as possible. Smoother surfaces lead to increased static and kinetic friction and wear. In the case of heads used in conjunction with rigid disks, the dynamics of a head are very critical in maintaining head-disk separation. All magnetic media have to be lubricated sufficiently to minimize head and magnetic-medium wear. The lubrication is carried out either topically or in bulk. Disk/head interface tribology is the limiting factor in achieving maximum data storage density.
High magnetic storage density in modern disk drives is achieved by the use of very smooth thin-film rigid disks that allow ultra-low flying of read/write head sliders over the disk surface. However, smooth surfaces result in stiction (high static friction) during rest and high stiction/friction during the contact start/stop (CSS) operation, especially with the presence of a thin film of liquid lubricant or aidsorbed water vapor at the head-disk interface. Disk surfaces are therefore textured to minimize stiction/friction. There is a critical h/.sigma. (total liquid film thickness/standard deviation of surface heights) above which stiction increases rapidly with an increase in the liquid film thickness. Distribution of local roughness plays an important role in friction/stiction and wear. Thus optimization of roughness distribution on the disk surface is required. In most models, surface height distribution is assumed to follow a gaussian distribution. However, engineering surfaces are frequently non-gaussian with the degree of non-gaussian characteristics dependent upon materials and surface finishing processes used. For example, magnetic rigid disk surfaces used in the magnetic storage industry are highly non-gaussian. The use of a gaussian analysis in such cases can lead to erroneous results.
Contact of two rough surfaces at an interface occurs at a small fraction of the nominal area of contact. Real area of contact and interfacial adhesion primarily control the friction of an interface. With the presence of a thin film of liquid lubricant or adsorbed water layer at the interface, menisci form around the contacting and near-contacting asperities. The meniscus formation results in stiction problems in the head-medium interfaces. Stiction in rigid disk drives is intimately related to the ratio of the liquid film thickness (h) and the composite standard deviation of the surface heights of disk and head surfaces (.sigma.). For the same liquid film thickness, rougher disks (having a higher .sigma.) exhibit a lower stiction than smoother disks (Bhushan, 1990). Normally in disk drives, the stiction induced with meniscus bridges is a more serious problem than the kinetic friction during sliding. CSS operations result in wear of surface making surfaces smoother which increases stiction after use. There is a need therefore for a magnetic media surface which has optimum roughness which minimizes friction and stiction to reduce wear and prolong the life of the recording media.
Based on classical theory of friction (Bowden and Tabor, 1950) the kinetic friction is proportional to the real area of contact which is high for smoother surfaces (Greenwood and Williamson, 1966). For partially wet contacts, menisci bridges are formed at the interface which result in intrinsic attractive (meniscus) force leading to high static friction. The number of bridges and asperities increase for smoother surfaces leading to high static friction. Thus minimum kinetic and static friction occurs for two rough surfaces, viz. a rough slider against a rough disk. This however does riot satisfy the objective of achieving a high recording density, which is obtained by the use of smooth surfaces for the head and the disk. Thus optimization of the roughness distribution of head and disk surfaces is required in order to satisfy both objectives. Optimization of surface roughness also reduces wear.